| 摘要 |
A phase locked loop tuning system for a television receiver includes a reference oscillator, a reference divider for dividing the frequency of the output signal of the reference oscillator, a voltage controlled local oscillator, a prescaler for dividing the frequency of the output signal of the local oscillator by a predetermined factor, a programmable divider for dividing the frequency of the output signal of the prescaler by a factor determined by the presently selected channel, a phase comparator for generating a series of pulses representing the phase and frequency relationships between the output signal of the reference divider and the output signal of the programmable divider and a low pass filter for deriving a control voltage for the local oscillator in response to the pulses generated by the phase comparator. The pulses generated by the phase comparator have polarities and widths dependent on the sense of the phase and/or frequency differences between the output signals of the reference divider and the programmable divider. A lock detector determines whether the pulses generated by the phase comparator have widths less than a first predetermined width at the end of a predetermined time. If the output signal of the prescaler has an incorrect frequency in response to a temporary malfunction of a component within the loop, the pulses generated by the phase comparator will not have widths less than the first predetermined width. In response to this occurrence, a mode switch decouples the pulses generated by the phase comparator from the low pass filter and couples pulses having a predetermined duty cycle, a predetermined repetition rate and the opposite polarity to the low pass filter instead. When the lock detector determines the first time one of its output pulses has a width less than a second predetermined width, narrower than the first, the pulses generated by the phase comparator are again coupled to the low pass filter. |
